Concept 03 / 05Environmental Chemistry
Atmospheric Pollution: Gaseous Pollutants and Acid Rain
Structure of the atmosphere, environmental pollution and pollutants, tropospheric gaseous pollutants: oxides of sulfur and nitrogen, hydrocarbons, carbon monoxide and carbon dioxide with their sources and effects, and the formation, chemistry and harmful effects of acid rain.
Environmental chemistry studies the origin, transport, reactions, effects and fate of chemical species in the environment. A pollutant is any substance present in the wrong place, at the wrong time or in the wrong concentration, harming living things. Pollutants may be biodegradable (sewage) or non-biodegradable (DDT, heavy metals, plastics).
The atmosphere. The troposphere (up to about ) contains the air we breathe, water vapour and clouds; the stratosphere ( to ) contains the ozone layer. Tropospheric pollution is caused by gaseous pollutants and particulates.
Gaseous air pollutants.
Oxides of sulfur (, ): from burning sulfur-containing fossil fuels and smelting sulfide ores. causes respiratory problems (asthma, bronchitis) and irritates eyes; even low concentrations damage plants (stiffening of flower buds). It is oxidised to in air, catalysed by particulates: .
Oxides of nitrogen: at the high temperatures of engines and power plants, (); NO is oxidised to (), which is rapid with ozone (). irritates lungs, damages leaves (leaf fall) and slows photosynthesis; it is a key ingredient of photochemical smog.
Hydrocarbons: unburnt fuel from vehicles; some are carcinogenic, and they drive smog chemistry.
Carbon monoxide: from incomplete combustion (vehicle exhaust, badly adjusted engines, forest fires, cigarette smoke). It binds haemoglobin about times more strongly than oxygen, forming carboxyhaemoglobin; at to carboxyhaemoglobin the oxygen-carrying capacity drops noticeably, causing headache, weak eyesight and nervousness; higher levels are fatal.
Carbon dioxide: from respiration and combustion; not toxic at normal levels, but its rising concentration increases the greenhouse effect.
Acid rain (see figure). Normal rain water has pH about , because dissolved forms carbonic acid (). When the pH falls below , it is acid rain. and are oxidised and dissolve in water:
; .
Effects: leaches nutrients from soil and harms crops and forests; releases toxic metals (Al, Pb, Cu) into water; kills fish in lakes; corrodes metals, marble and limestone buildings (), as seen in the yellowing of the Taj Mahal from refinery emissions near Agra.
Control: low-sulfur fuels, scrubbing from flue gases, catalytic converters, planting trees, liming acidified lakes.
Key Takeaways:
- Normal rain pH ; acid rain below , from and .
- CO binds haemoglobin about times more strongly than .
- NO forms in hot engines and is oxidised to .
- Acid rain damages marble ("marble cancer"), soil, forests and aquatic life.
- Pitfall: is not the cause of acid rain; it only sets the natural pH of rain at .
Brain-Teaser 1: Rain in an industrial area has pH . How many times more acidic is it than normal rain?
Solution: Normal rain has pH . A difference of one pH unit means ten times the concentration: times more acidic.
Brain-Teaser 2: A power plant burns of coal containing sulfur by mass per day. If all the sulfur ends up as sulfuric acid in rain, what mass of forms daily?
Solution: Sulfur . Each mole of S gives one mole of (): , about tonnes of sulfuric acid per day.
Brain-Teaser 3: Why is CO dangerous even though it is odourless and non-irritating, and why are catalytic converters fitted to cars?
Solution: CO displaces oxygen from haemoglobin (forming stable carboxyhaemoglobin), starving tissues of oxygen without any warning signs. Catalytic converters (Pt, Pd, Rh) oxidise CO and unburnt hydrocarbons to and water and reduce NO to , cutting three major pollutants at once.
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